A. Field of the Invention
The present invention relates to a circuit for detecting a foreign potential on telephone lines and, more particularly, to a power-cross detection circuit used for protecting telephone systems.
B. Background of Related Art
Any terminal device connected to a telephone line must undergo a "power-cross" test. This test is performed in order to verify the ability of the terminal to detect the presence of a foreign potential such as a high voltage of up to 1000 volts a-c from the local power company which may become crossed with the telephone line in the environment outside a telephone plant. FIG. 1 shows a telephone 10 connected to a telephone plant 12 through wires 14. A power line 16 poses a threat to a telephone plant 12 and any personnel operating the equipment. The test includes forcing a 1 ampere current into tip and ring terminals 18, 20 regardless of the state (i.e. talking, standby or ringing) of the line and checking whether the plant 12 survives the test. Generally, to provide a 1 ampere current, a 1,000 volt, 60 hertz signal through a 1 k ohm resister is applied to the tip and ring terminals 18, 20. Other combinations of voltages and resistance values, such as a 600 volts signal through a 600 ohms resistor, or a 50 volts signal through a 50 ohms resistor could also be used.
As mentioned above, the power-cross test is applied regardless of the state of the plant 12 i.e., a person could be talking on the telephone, the telephone could be in a standby mode, or the telephone could be ringing. In known techniques, during either the talking mode or the standby mode, as shown in FIGS. 1 and 2, a protection device such as a diode, SCR, or a neon lamp 22 is connected to terminals 18 and 20 so that when a high voltage such as the 118 volts a-c power signal crosses wires 14 (FIG. 1), the current goes through the protection device 22, thereby preventing the telephone plant 12 or telephone 10 from being damaged.
As shown in FIG. 3, during the ringing mode, the telephone 10 is connected to ground via the tip terminal 18, whereas the ring terminal 20 is connected to a ringer voltage RV for performing the ringing operation of telephone 10. In this situation, the protection device 22 is disconnected. Thus, although there is some kind of protection provided in the talking and standby modes, there is no protection provided if the power-cross test is conducted in the ringing mode.